The goal of this protocol is to define the abnormalities in pancreatic islet B-cell responsiveness to glucose and other stimuli in infants and children with different genetic disorders which cause hyperinsulinism. The most severe form of the disease has recently been linked to the sulfonylurea receptor (SUR). In patients and their carrier parents, studies will determine the acute insulin responses (AIR) to glucose, tolbutamide, and other secretogogoues as well as the insulin responses to graded hyperglycemic stimulation.